Carsten Deibel's articles on arXiv2017-08-18T00:00:00-04:00http://arxiv.org/a/deibel_c_1http://arxiv.org/abs/1605.03526v12016-05-11T13:29:16-04:002016-05-11T13:29:16-04:00Charge Carrier Concentration Dependence of Encounter-Limited Bimolecular Recombination in Phase-Separated Organic Semiconductor BlendsUnderstanding how the complex intermolecular- and nano-structure present in organic semiconductor donor-acceptor blends impacts charge carrier motion, interactions, and recombination behavior is a critical fundamental issue with a particularly major impact on organic photovoltaic applications. In this study, kinetic Monte Carlo (KMC) simulations are used to numerically quantify the complex bimolecular charge carrier recombination behavior in idealized phase-separated blends. Recent KMC simulations have identified how the encounter-limited bimolecular recombination rate in these blends deviates from the often used Langevin model and have been used to construct the new power mean mobility model. Here, we make a challenging but crucial expansion to this work by determining the charge carrier concentration dependence of the encounter-limited bimolecular recombination coefficient. In doing so, we find that an accurate treatment of the long-range electrostatic interactions between charge carriers is critical, and we further argue that many previous KMC simulation studies have used a Coulomb cutoff radius that is too small, which causes a significant overestimation of the recombination rate. To shed more light on this issue, we determine the minimum cutoff radius required to reach an accuracy of less than $\pm10\%$ as a function of the domain size and the charge carrier concentration and then use this knowledge to accurately quantify the charge carrier concentration dependence of the recombination rate. Using these rigorous methods, we finally show that the parameters of the power mean mobility model are determined by a newly identified dimensionless ratio of the domain size to the average charge carrier separation distance.Michael C. HeiberThuc-Quyen NguyenCarsten Deibel10.1103/PhysRevB.93.205204Main article: 9 pages, 5 figures, Supplementary Information: 3 pages, 1 figurePhys. Rev. B 93, 205204 (2016)http://arxiv.org/abs/1503.02848v12015-03-10T06:23:38-04:002015-03-10T06:23:38-04:00Encounter-Limited Charge Carrier Recombination in Phase Separated Organic Semiconductor BlendsThe theoretical effects of phase separation on encounter-limited charge carrier recombination in organic semiconductor blends are investigated using kinetic Monte Carlo (KMC) simulations of pump-probe experiments. Using model bulk heterojunction morphologies, the dependence of the recombination rate on domain size and charge carrier mobility are quantified. Unifying competing models and simulation results, we show that the mobility dependence of the recombination rate can be described using the power mean of the electron and hole mobilities with a domain size dependent exponent. Additionally, for domain sizes typical of organic photovoltaic devices, we find that phase separation reduces the recombination rate by less than one order of magnitude compared to the Langevin model and that the mobility dependence can be approximated by the geometric mean.Michael C. HeiberChristoph BaumbachVladimir DyakonovCarsten Deibel10.1103/PhysRevLett.114.136602Main article: 6 pages, 4 figures, Supplementary Information: 3 pages, 1 figurePhys. Rev. Lett. 114, 136602 (2015)http://arxiv.org/abs/1209.1922v52014-04-30T07:52:48-04:002012-09-10T05:51:02-04:00Distribution of charge carrier transport properties in organic semiconductors with Gaussian disorderThe charge carrier drift mobility in disordered semiconductors is commonly graphically extracted from time-of-flight (ToF) photocurrent transients yielding a single transit time. However, the term transit time is ambiguously defined and fails to deliver a mobility in terms of a statistical average. Here, we introduce an advanced computational procedure to evaluate ToF transients, which allows to extract the whole distribution of transit times and mobilities from the photocurrent transient, instead of a single value. This method, extending the work of Scott et al. (Phys. Rev. B 46, 8603), is applicable to disordered systems with a Gaussian density of states (DOS) and its accuracy is validated using one-dimensional Monte Carlo simulations. We demonstrate the superiority of this new approach by comparing it to the common geometrical analysis of hole ToF transients measured on poly(3-hexyl thiophene-2,5-diyl) (P3HT). The extracted distributions provide access to a very detailed and accurate analysis of the charge carrier transport. For instance, not only the mobility given by the mean transit time, but also the mean mobility can be calculated. Whereas the latter determines the macroscopic photocurrent, the former is relevant for an accurate determination of the energetic disorder parameter $\sigma$ within the Gaussian disorder model (GDM). $\sigma$ derived by using the common geometrical method is, as we show, underestimated instead.Jens LorrmannManuel RufDavid VockeVladimir DyakonovCarsten Deibel10.1063/1.487568310 pages, 6 figuresJ. Appl. Phys. 115, 183702 (2014)http://arxiv.org/abs/1403.7466v22014-04-07T10:31:31-04:002014-03-28T14:02:53-04:00Nongeminate recombination in neat P3HT and P3HT:PCBM blend filmsThe slow decay of charge carriers in polymer-fullerene blends measured in transient studies has raised a number of questions about the mechanisms of nongeminate recombination in these systems. In an attempt to understand this behavior, we have applied a combination of steady-state and transient photoinduced absorption measurements to compare nongeminate recombination behavior in films of neat poly(3-hexyl thiophene) (P3HT) and P3HT blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Transient measurements show that carrier recombination in the neat P3HT film exhibits second-order decay with a recombination rate coefficient that is similar to that predicted by Langevin theory. In addition, temperature dependent measurements indicate that neat films exhibit recombination behavior consistent with the Gaussian disorder model. In contrast, the P3HT:PCBM blend films are characterized by a strongly reduced recombination rate and an apparent recombination order greater than two. We then assess a number of previously proposed explanations for this behavior, including phase separation, carrier concentration dependent mobility, non-encounter limited recombination, and interfacial states. In the end, we propose a model in which pure domains with a Gaussian density of states are separated by a mixed phase with an exponential density of states. We find that such a model can explain both the reduced magnitude of the recombination rate and the high order recombination kinetics and, based on the current state of knowledge, is the most consistent with experimental observations.Julien GorenflotMichael C. HeiberAndreas BaumannJens LorrmannMatthias GunzAndreas KämpgenVladimir DyakonovCarsten Deibel10.1063/1.48708059 pages, 4 figures; corrected a few minor typos and grammatical errorsJ. Appl. Phys. 115, 144502 (2014)http://arxiv.org/abs/1309.1107v12013-09-04T13:13:16-04:002013-09-04T13:13:16-04:00Effects of Characteristic Length Scales on the Exciton Dynamics in Rubrene Single CrystalsAs for its inorganic counterparts the future developments in organic electronics are driven by an advanced device miniaturization. Therefore, the opto-electronic behavior of up-to-date devices is progressively governed by the local structural environment. However, there is a lack of organic semiconductor materials providing access to the fundamental structure-functionality relation, either due to limitations by their inherent growth or their optical characteristics. In this work we present a systematic investigation of the optical states, so-called excitons, and their temporal evolution in the prototypical organic semiconductor rubrene by means of time and temperature dependent photoluminescence studies. This material offers the unique possibility of preparing well-defined morphologies with adjustable degree of confinement. By this approach we are able to confirm the direct influence on the temperature dependent optical processes with picosecond resolution already for a spatial localization of excitation on the \mu m length scale. While in bulk single crystals the exciton decay dynamics are governed by thermally activated singlet fission, excitons created in microcrystals are trapped by dark states localized on the surface and leading to a pronounced enhancement of their average lifetime. Our results highlight the impact of the local environment on the excitonic states and their dynamics in organic semiconductors. With respect to the spatial dimensions of organic thin film devices, this correlation and the reported effects emerging by the confinement have to be considered upon further miniaturization and in the development of innovative device concepts, such as photovoltaic cells based on triplet-harvesting.Björn GiesekingTeresa SchmeilerBenjamin MüllerCarsten DeibelBernd EngelsVladimir DyakonovJens Pflaum10.1103/PhysRevB.90.20530512 pages, 3 figures, 1 table, supplementary: 6 pages, 4 figuesPhys. Rev. B 90, 205305 (2014)http://arxiv.org/abs/1302.0367v32013-07-10T03:53:03-04:002013-02-02T06:33:24-05:00Order of Decay of Mobile Charge Carriers in P3HT:PCBM Solar CellsThe charge carrier dynamics of organic solar cells are strongly influenced by trapping and allow to draw conclusions on the loss mechanisms limiting the photovoltaic performance. In this study we derive the recombination order \Delta\ of mobile charge carriers. For annealed P3HT:PCBM solar cells, it allows us to pinpoint the dominant recombination of mobile with trapped charge carriers in tail states. While the characteristic tail state energy of about 40 meV rises to about 100 meV for 30 h oxygen exposure under illumination, \Delta\ decreases only weakly from 1.70 to 1.62: This corresponds to a slight shift towards trap-assisted recombination.Carsten DeibelDaniel RauhAlexander Foertig10.1063/1.48167204 pages, 3 figures. Corrected one reference and one technical termAppl. Phys. Lett. 103, 043307 (2013)http://arxiv.org/abs/1207.3341v22012-08-22T09:13:28-04:002012-07-13T15:55:37-04:00Shockley Equation Parameters of Organic Solar Cells derived by Transient TechniquesThe Shockley equation (SE), originally derived to describe a p--n junction, was frequently used in the past to simulate current--voltage (j/V) characteristics of organic solar cells (OSC). In order to gain a more detailed understanding of recombination losses, we determined the SE parameters, i.e. the ideality factor and the dark saturation current, from temperature dependent static j/V-measurements on poly(3-hexylthiophene-2,5-diyl)(P3HT):[6,6]-phenyl-C$_{61}$ butyric acid methyl ester (PCBM) bulk heterojunction solar cells. As we show here, these parameters are directly related to charge carrier recombination and become also accessible by transient photovoltage and photocurrent methods in the case of field-independent charge carrier generation. Although determined in very different ways, both SE parameters were found to be identical. The good agreement of static and transient approaches over a wide temperature range demonstrates the validity of the Shockley model for OSC based on material systems satisfying the requirement of field-independent polaron-pair dissociation. In particular, we were able to reproduce the photocurrent at various light intensities and temperatures from the respective nongeminate recombination rates. Furthermore, the temperature dependence of the dark saturation current $j_0$ allowed determining the effective band gap of the photoactive blend perfectly agreeing with the literature values of the energy onset of the photocurrent due to charge transfer absorption. We also present a consistent model directly relating the ideality factor to recombination of free with trapped charge carriers in an exponential density of tail states. We verify this finding by data from thermally stimulated current measurements.A. FoertigJ. RauhV. DyakonovC. Deibel10.1103/PhysRevB.86.1153028 pages, 6 figuresPhys. Rev. B 86, 115302 (2012)http://arxiv.org/abs/1207.3787v22012-07-18T04:54:18-04:002012-07-14T07:20:17-04:00Impact of nongeminate recombination on the performance of pristine and annealed P3HT:PCBM solar cellsTransient photovoltage (TPV) and voltage dependent charge extraction (CE) measurements were applied to poly(3-hexylthiophene)(P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) bulk heterojunction solar cells to analyze the limitations of solar cell performance in pristine and annealed devices. From the determined charge carrier decay rate under open circuit conditions and the voltage dependent charge carrier densities n(V) the nongeminate loss current jloss of the device is accessible. We found that jloss alone is sufficient to describe the j-V characteristics across the whole operational range, for annealed and, not yet shown before, also for the lower performing pristine solar cells. Even in a temperature range from 300 K to 200 K nongeminate recombination is found to be the dominant and, therefore, performance limiting loss process. Consequently, charge photogeneration is voltage independent in the voltage range studied.Markus GlueckerAlexander FoertigVladimir DyakonovCarsten Deibel10.1002/pssr.2012062483 pages, 3 figures; Rapid Research Letter 2012Phys. Status Solidi (RRL) 6, 337 (2012)http://arxiv.org/abs/1206.4529v12012-06-20T11:22:53-04:002012-06-20T11:22:53-04:00Excitation Dynamics in Low Band Gap Donor-Acceptor Copolymers and BlendsDonor-acceptor (D-A) type copolymers show great potential for the application in the active layer of organic solar cells. Nevertheless the nature of the excited states, the coupling mechanism and the relaxation pathways following photoexcitation are yet to be clarified. We carried out comparative measurements of the steady state absorption and photoluminescence (PL) on the copolymer poly[N-(1-octylnonyl)-2,7-carbazole] -alt-5,5-[4',7' -di(thien-2-yl)-2',1',3' -benzothiadiazole] (PCDTBT), its building blocks as well as on the newly synthesized N-(1-octylnonyl)-2,7-bis-[(5-phenyl)thien-2-yl)carbazole (BPT-carbazole) (see Figure 1). The high-energy absorption band (HEB) of PCDTBT was identified with absorption of carbazoles with adjacent thiophene rings while the low-energy band (LEB) originates instead from the charge transfer (CT) state delocalized over the aforementioned unit with adjacent benzothiadiazole group. Photoexcitation of the HEB is followed by internal relaxation prior the radiative decay to the ground state. Adding PC70BM results in the efficient PL quenching within the first 50 ps after excitation. From the PL excitation experiments no evidence for a direct electron transfer from the HEB of PCDTBT towards the fullerene acceptor was found, therefore the internal relaxation mechanisms within PCDTBT can be assumed to precede. Our findings indicate that effective coupling between copolymer building blocks governs the photovoltaic performance of the blends.Björn GiesekingBerthold JäckEduard PreisStefan JungMichael ForsterUllrich ScherfCarsten DeibelVladimir Dyakonov10.1002/aenm.2012003047 pages, 6 figures, 1 tableAdv. Ener. Mater. 2, 1477 (2012)http://arxiv.org/abs/1206.2808v12012-06-13T09:53:05-04:002012-06-13T09:53:05-04:00Investigation of electronic trap states in organic photovoltaic materials by current-based deep level transient spectroscopyCurrent-based deep level transient spectroscopy was used to study trap states in poly(3- hexylthiophene-2,5-diyl) (P3HT), [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) and P3HT:PCBM blend. The obtained spectra showed traps of 87 meV activation energy in pure P3HT and 21 meV for PCBM. The blend shows a complex emission rate spectrum consisting of several different emission rate bands in the range of (0.1-30) s^-1, yielding activation energies between about 30 meV and 160 meV.Stefan NeugebauerJulia RauhCarsten DeibelVladimir Dyakonov10.1063/1.47316374 pages, 3 figuresAppl. Phys. Lett. 100, 263304 (2012)http://arxiv.org/abs/1204.4788v12012-04-21T04:17:37-04:002012-04-21T04:17:37-04:00Direct and charge transfer mediated photogeneration in polymer-fullerene bulk heterojunction solar cellsWe investigated photogeneration yield and recombination dynamics in blends of poly(3-hexyl thiophene) (P3HT) and poly[2-methoxy-5-(30,70-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) with [6,6]- phenyl-C61 butyric acid methyl ester (PC61BM) by means of temperature dependent time delayed collection field (TDCF) measurements. In MDMO-PPV:PC61BM we find a strongly field dependent polaron pair dissociation which can be attributed to geminate recombination in the device. Our findings are in good agreement with field dependent photoluminescence measurements published before, supporting a scenario of polaron pair dissociation via an intermediate charge transfer (CT) state. In contrast, polaron pair dissociation in P3HT:PC61BM shows only a very weak field dependence, indicating an almost field independent polaron pair dissociation or a direct photogeneration. Furthermore, we found Langevin recombination for MDMO-PPV:PC61BM and strongly reduced Langevin recombination for P3HT:PC61BM.Markus MingebachStefan WalterVladimir DyakonovCarsten Deibel10.1063/1.47118494 pages, 3 figuresAppl. Phys. Lett. 100, 193302 (2012)http://arxiv.org/abs/1203.6105v12012-03-27T19:14:22-04:002012-03-27T19:14:22-04:00Charge density dependent nongeminate recombination in organic bulk heterojunction solar cellsApparent recombination orders exceeding the value of two expected for bimolecular recombination have been reported for organic solar cells in various publications. Two prominent explanations are bimolecular losses with a carrier concentration dependent prefactor due to a trapping limited mobility, and protection of trapped charge carriers from recombination by a donor--acceptor phase separation until reemission from these deep states. In order to clarify which mechanism is dominant we performed temperature and illumination dependent charge extraction measurements under open circuit as well as short circuit conditions at poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C$_{61}$butyric acid methyl ester (P3HT:PC$_{61}$BM) and PTB7:PC$_{71}$BM (Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]) solar cells in combination with current--voltage characteristics. We show that the charge carrier density $n$ dependence of the mobility $\mu$ and the recombination prefactor are different for PC$_{61}$BM at temperatures below 300K and PTB7:PC$_{71}$BM at room temperature. Therefore, in addition to $\mu(n)$ a detrapping limited recombination in systems with at least partial donor--acceptor phase separation is required to explain the high recombination orders.D. RauhC. DeibelV. Dyakonov10.1002/adfm.2011031187 pages, 4 figuresAdv. Funct. Mater. 22, 3371 (2012)http://arxiv.org/abs/1110.1316v22011-12-30T15:07:30-05:002011-10-06T12:42:20-04:00Reversible and Irreversible Interactions of Poly(3-hexylthiophene) with Oxygen Studied by Spin-Sensitive MethodsUnderstanding of degradation mechanisms in polymer:fullerene bulk-heterojunctions on the microscopic level aimed at improving their intrinsic stability is crucial for the breakthrough of organic photovoltaics. These materials are vulnerable to exposure to light and/or oxygen, hence they involve electronic excitations. To unambiguously probe the excited states of various multiplicities and their reactions with oxygen, we applied combined magneto-optical methods based on multifrequency (9 and 275 GHz) electron paramagnetic resonance (EPR), photoluminescence (PL), and PL-detected magnetic resonance (PLDMR) to the conjugated polymer poly(3-hexylthiophene) (P3HT) and polymer:fullerene bulk heterojunctions (P3HT:PCBM; PCBM = [6,6]-phenyl-C61-butyric acid methyl ester). We identified two distinct photochemical reaction routes, one being fully reversible and related to the formation of polymer:oxygen charge transfer complexes, the other one, irreversible, being related to the formation of singlet oxygen under participation of bound triplet excitons on the polymer chain. With respect to the blends, we discuss the protective effect of the methanofullerenes on the conjugated polymer bypassing the triplet exciton generation.Andreas SperlichHannes KrausCarsten DeibelHubert BlokJan SchmidtVladimir Dyakonov10.1021/jp2077215J. Phys. Chem. B 2011, 115, 13513-13518http://arxiv.org/abs/1110.2671v12011-10-12T10:42:23-04:002011-10-12T10:42:23-04:00Binding energy of singlet excitons and charge transfer complexes in MDMO-PPV:PCBM solar cellsThe influence of an external electric field on the photoluminescence intensity of singlet excitons and charge transfer complexes is investigated for a poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) diode and a bulk heterojunction of the PPV in combination with [6,6]-phenyl-C61 butyric acid methylester (PCBM), respectively. The experimental data is related to the dissociation probability derived from the Onsager-Braun model. In this way, a lower limit for the singlet exciton binding energy of MDMO-PPV is determined as (327 +- 30) meV, whereas a significantly lower value of (203 +- 18) meV is extracted for the charge transfer complex in a MDMO-PPV:PCBM blend.Julia KernSebastian SchwabCarsten DeibelVladimir Dyakonov10.1002/pssr.2011054303 pages, 4 figures, published online at physica status solidi (RRL)Phys. Status Solidi (RRL) Volume 5, Issue 10-11, pages 364-366, November 2011http://arxiv.org/abs/1110.1577v12011-10-07T12:23:04-04:002011-10-07T12:23:04-04:00Spin Signatures of Photogenerated Radical Anions in Polymer-[70]Fullerene Bulk Heterojunctions: High Frequency Pulsed EPR SpectroscopyCharged polarons in thin films of polymer-fullerene composites are investigated by light-induced electron paramagnetic resonance (EPR) at 9.5 GHz (X-band) and 130 GHz (D-band). The materials studied were poly(3-hexylthiophene) (PHT), [6,6]-phenyl-C61-butyric acid methyl ester (C60-PCBM), and two different soluble C70-derivates: C70-PCBM and diphenylmethano[70]fullerene oligoether (C70-DPM-OE). The first experimental identification of the negative polaron localized on the C70-cage in polymer-fullerene bulk heterojunctions has been obtained. When recorded at conventional X-band EPR, this signal is overlapping with the signal of the positive polaron, which does not allow for its direct experimental identification. Owing to the superior spectral resolution of the high frequency D-band EPR, we were able to separate light-induced signals from P+ and P- in PHT-C70 bulk heterojunctions. Comparing signals from C70-derivatives with different side-chains, we have obtained experimental proof that the polaron is localized on the cage of the C70 molecule.Oleg G. PoluektovSalvatore FilipponeNazario MartinAndreas SperlichCarsten DeibelVladimir Dyakonov10.1021/jp1012347J. Phys. Chem. B, 2010, 114, 14426http://arxiv.org/abs/1110.0658v12011-10-04T08:34:50-04:002011-10-04T08:34:50-04:00Detailed study of N,N'-(diisopropylphenyl)- terrylene-3,4:11,12-bis(dicarboximide) as electron acceptor for solar cells applicationWe report on terrylene-3,4:11,12-bis(dicarboximide) (TDI) as electron acceptor for bulk-heterojunction solar cells using poly(3-hexyl thiophene) (P3HT) as complementary donor component. Enhanced absorption was observed in the blend compared to pure P3HT. As shown by the very efficient photoluminescence (PL) quenching, the generated excitons are collected at the interface between the donor and acceptor, where they separate into charges which we detect by photoinduced absorption and electron-spin resonance (ESR). Time-of-flight (TOF) photoconductivity measurements reveal a good electron mobility of 10-3 cm2 V-1 s-1 in the blend. Nevertheless, the photocurrent in solar cells was found to be surprisingly low. Supported by the external quantum efficiency (EQE) spectrum as well as morphological studies by way of X-ray diffraction and atomic force microscopy, we explain our observation by the formation of a TDI hole blocking layer at the anode interface which prevents the efficiently generated charges to be extracted.Julien GorenflotAndreas SperlichAndreas BaumannDaniel RauhAleksey VasilevChen LiMartin BaumgartenCarsten DeibelVladimir Dyakonov10.1016/j.synthmet.2011.09.041Original research article, 9 pages, 10 figures, 1 tableSynth. Metals 162, 2669 (2012)http://arxiv.org/abs/1110.0588v12011-10-04T03:05:06-04:002011-10-04T03:05:06-04:00Relation of open circuit voltage to charge carrier density in organic bulk heterojunction solar cellsThe open circuit voltage Voc and the corresponding charge carrier density were measured in dependence of temperature and illumination intensity by current-voltage and charge extraction measurements for P3HT:PCBM and P3HT:bisPCBM solar cells. At lower temperatures a saturation of Voc was observed which can be explained by energetic barriers at the contacts (metal-insulator-metal model). Such injection barriers can also influence Voc at room temperature and limit the performance of the working solar cell, as was assured by macroscopic device simulations on temperature-dependent IV characteristics. However, under most conditions - room temperature and low barriers - Voc is given by the effective bandgap.Daniel RauhAlexander WagenpfahlCarsten DeibelVladimir Dyakonov10.1063/1.35669794 pages, 2 figuresAppl. Phys. Lett. 98:133301 (2011)http://arxiv.org/abs/1109.5528v12011-09-26T07:38:52-04:002011-09-26T07:38:52-04:00Built-in potential and validity of Mott--Schottky analysis in organic bulk heterojunction solar cellsWe investigated poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester bulk heterojunction (BHJ) solar cells by means of pulsed photocurrent, temperature dependent current-voltage and capacitance-voltage measurements. We show that a direct transfer of Mott-Schottky (MS) analysis from inorganic devices to organic BHJ solar cells is not generally appropriate to determine the built-in potential, since the resulting potential depends on the active layer thickness. Pulsed photocurrent measurements enabled us to directly study the case of quasi flat bands (QFB) in the bulk of the solar cell. It is well below the built-in potential and differs by diffusion-induced band-bending at the contacts. In contrast to MS analysis the corresponding potential is independent on the active layer thickness and therefore a better measure for flat band conditions in the bulk of a BHJ solar cell as compared to MS analysis.Markus MingebachCarsten DeibelVladimir Dyakonov10.1103/PhysRevB.84.1532014 pages, 5 figuresPhys. Rev. B 84, 153201 (2011)http://arxiv.org/abs/1107.3525v12011-07-18T14:53:13-04:002011-07-18T14:53:13-04:00Triplet Exciton Generation in Bulk-Heterojunction Solar Cells based on Endohedral FullerenesOrganic bulk-heterojunctions (BHJ) and solar cells containing the trimetallic nitride endohedral fullerene 1-[3-(2-ethyl)hexoxy carbonyl]propyl-1-phenyl-Lu3N@C80 (Lu3N@C80-PCBEH) show an open circuit voltage (VOC) 0.3 V higher than similar devices with [6,6]-phenyl-C[61]-butyric acid methyl ester (PC61BM). To fully exploit the potential of this acceptor molecule with respect to the power conversion efficiency (PCE) of solar cells, the short circuit current (JSC) should be improved to become competitive with the state of the art solar cells. Here, we address factors influencing the JSC in blends containing the high voltage absorber Lu3N@C80-PCBEH in view of both photogeneration but also transport and extraction of charge carriers. We apply optical, charge carrier extraction, morphology, and spin-sensitive techniques. In blends containing Lu3N@C80-PCBEH, we found 2 times weaker photoluminescence quenching, remainders of interchain excitons, and, most remarkably, triplet excitons formed on the polymer chain, which were absent in the reference P3HT:PC61BM blends. We show that electron back transfer to the triplet state along with the lower exciton dissociation yield due to intramolecular charge transfer in Lu3N@C80-PCBEH are responsible for the reduced photocurrent.Moritz LiedtkeAndreas SperlichHannes KrausAndreas BaumannCarsten DeibelMaarten J. M. WirixJoachim LoosClaudia M. CardonaVladimir Dyakonov10.1021/ja2025432J. Am. Chem. Soc., 2011, 133 (23), pp 9088--9094http://arxiv.org/abs/1107.3381v12011-07-18T04:46:23-04:002011-07-18T04:46:23-04:00Electronic Trap States in MethanofullerenesThe trap states in three fullerene derivatives, namely PC61BM ([6,6]-phenyl C61 butyric acid methyl ester), bisPC61BM (bis[6,6]-phenyl C61 butyric acid methyl ester) and PC71BM ([6,6]-phenyl C71 butyric acid methyl ester), are investigated by thermally stimulated current measurements (TSC). Thereby, the lower limit of the trap densities for all studied methanofullerenes exhibits values in the order of 10^22 m^-3 with the highest trap density in bisPC61BM and the lowest in PC61BM. Fractional TSC measurements on PC61BM reveal a broad trap distribution instead of discrete trap levels with activation energies ranging from 15 meV to 270 meV and the maximum at about 75 meV. The activation energies of the most prominent traps in the other two fullerene derivatives are significantly higher, being at 96 meV and 223 meV for PC71BM and 184 meV for bisPC61BM, respectively. The influence of these findings on the performance of organic solar cells is discussed.Julia SchafferhansCarsten DeibelVladimir Dyakonov10.1002/aenm.2011001757 pages, 5 figures, 1 tableAdvanced Energy Materials, 1, 655, (2011)http://arxiv.org/abs/1107.1576v12011-07-08T04:45:44-04:002011-07-08T04:45:44-04:00Influence of Phase Segregation on Recombination Dynamics in Organic Bulk-Heterojunction Solar CellsWe studied the recombination dynamics of charge carriers in organic bulk heterojunction solar cells made of the blend system poly(2,5-bis(3-dodecyl thiophen-2-yl) thieno[2,3-b]thiophene) (pBTCT-C12):[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) with a donor--acceptor ratio of 1:1 and 1:4. The techniques of charge carrier extraction by linearly increasing voltage (photo-CELIV) and, as local probe, time-resolved microwave conductivity (TRMC) were used. We observed a difference in the initially extracted charge carrier concentration in the photo-CELIV experiment by one order of magnitude, which we assigned to an enhanced geminate recombination due to a fine interpenetrating network with isolated phase regions in the 1:1 pBTCT-C12:PC61BM bulk heterojunction solar cells. In contrast, extensive phase segregation in 1:4 blend devices leads to an efficient polaron generation resulting in an increased short circuit current density of the solar cell. For both studied ratios a bimolecular recombination of polarons was found using the complementary experiments. The charge carrier decay order of above two for temperatures below 300 K can be explained by a release of trapped charges. This mechanism leads to a delayed bimolecular recombination processes. The experimental findings can be generalized to all polymer:fullerene blend systems allowing for phase segregation.Andreas BaumannTom J. SavenijeDharmapura Hanumantharaya K. MurthyMartin HeeneyVladimir DyakonovCarsten Deibel10.1002/adfm.20100235814 pages, 5 figuresAdv. Func. Mat. 21, 1687 (2011)http://arxiv.org/abs/1102.4289v12011-02-21T12:51:28-05:002011-02-21T12:51:28-05:00First principles calculations of anisotropic charge carrier mobilities in organic semiconductor crystalsThe orientational dependence of charge carrier mobilities in organic semiconductor crystals and the correlation with the crystal structure are investigated by means of quantum chemical first principles calculations combined with a model using hopping rates from Marcus theory. A master equation approach is presented which is numerically more efficient than the Monte Carlo method frequently applied in this context. Furthermore, it is shown that the widely used approach to calculate the mobility via the diffusion constant along with rate equations is not appropriate in many important cases. The calculations are compared with experimental data, showing good qualitative agreement for pentacene and rubrene. In addition, charge transport properties of core-fluorinated perylene bisimides are investigated.V. StehrJ. PfisterR. F. FinkB. EngelsC. Deibel10.1103/PhysRevB.83.15520815 pages, 18 figuresPhys. Rev. B 82, 155208 (2011)http://arxiv.org/abs/1102.1899v12011-02-09T11:39:30-05:002011-02-09T11:39:30-05:00Photoinduced C70 radical anions in polymer:fullerene blendsPhotoinduced polarons in solid films of polymer-fullerene blends were studied by photoluminescence (PL), photoinduced absorption (PIA) and electron spin resonance (ESR). The donor materials used were P3HT and MEH-PPV. As acceptors we employed PC60BM as reference and various soluble C70-derivates: PC70BM, two different diphenylmethano-[70]fullerene oligoether (C70-DPM-OE) and two dimers, C70-C70 and C60-C70. Blend films containing C70 revealed characteristic spectroscopic signatures not seen with C60. Light-induced ESR showed signals at g\geq2.005, assigned to an electron localized on the C70 cage. The formation of C70 radical anions also leads to a subgap PIA band at 0.92 eV, hidden in the spectra of C70-based P3HT and MEH-PPV blends, which allows for more exact studies of charge separated states in conjugated polymer:C70 blends.Andreas SperlichMoritz LiedtkeJulia KernHannes KrausCarsten DeibelSalvatore FilipponeJuan Luis DelgadoNazario MartínVladimir Dyakonov10.1002/pssr.2011050303 pages, 4 figures, accepted for PSS RRLPhys. Status Solidi RRL 5, No. 3, (2011) 128-130http://arxiv.org/abs/1011.6260v12010-11-29T10:26:24-05:002010-11-29T10:26:24-05:00Role of Polaron Pair Diffusion and Surface Losses in Organic Semiconductor DevicesBy applying Monte Carlo simulations we found that the extraction of bound polaron pairs (PP) at the electrodes is an important loss factor limiting the efficiency of organic optoelectronic and photovoltaic devices. Based upon this finding, we developed a unified analytic model consisting of exact Onsager theory, describing the dissociation of PP in organic donor-acceptor heterojunctions, the Sokel-Hughes model for the extraction of free polarons at the electrodes, as well as of PP diffusion leading to the aforementioned loss mechanism, which was not considered previously. Our approach allows to describe the simulation details on a macroscopic scale and to gain fundamental insights, which is important in view of developing an optimized photovoltaic device configuration.T. StrobelC. DeibelV. Dyakonov10.1103/PhysRevLett.105.2666024 pages, 4 figures. Accepted by Phys. Rev. LettPhys. Rev. Lett. 105, 266602 (2010)http://arxiv.org/abs/1006.4394v22010-10-29T04:30:23-04:002010-06-22T19:39:47-04:00Charge Carrier Extraction by Linearly Increasing Voltage:Analytic framework and ambipolar transientsUp to now the basic theoretical description of charge extraction by linearly increasing voltage (CELIV) is solved for a low conductivity approximation only. Here we present the full analytical solution, thus generalize the theoretical framework for this method. We compare the analytical solution and the approximated theory, showing that especially for typical organic solar cell materials the latter approach has a very limited validity. Photo-CELIV measurements on poly(3-hexyl thiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester based solar cells were then evaluated by fitting the current transients to the analytical solution. We found that the fit results are in a very good agreement with the experimental observations, if ambipolar transport is taken into account, the origin of which we will discuss. Furthermore we present parametric equations for the mobility and the charge carrier density, which can be applied over the entire experimental range of parameters.Jens LorrmannBekele Homa BadadaOlle InganäsVladimir DyakonovCarsten Deibel10.1063/1.35163928 pages, 5 figuresJ. Appl. Phys. 108, 113705 (2010)http://arxiv.org/abs/1006.4813v42010-10-26T18:15:55-04:002010-06-24T11:20:20-04:00Comment on "Interface state recombination in organic solar cells"In a recent paper, Street et al. [Phys. Rev. B 81, 205307 (2010)] propose first order recombination due to interface states to be the dominant loss mechanism in organic bulk heterojunction solar cells, based on steady-state current--voltage characteristics. By applying macroscopic simulations, we found that under typical solar cell conditions, monomolecular or bimolecular recombination cannot be inferred from the slope of the light intensity dependent photocurrent. In addition, we discuss the validity of calculating a mobility--lifetime product from steady-state measurements. We conclude that the experimental technique applied by Street et al. is not sufficient to unambiguously determine the loss mechanism.C. DeibelA. Wagenpfahl10.1103/PhysRevB.82.2073014 pages, 2 figures. Corrected Eqns. (2) and (4): 1/... was missingPhys. Rev. B 82, 207301 (2010)http://arxiv.org/abs/1005.5669v22010-09-10T05:33:49-04:002010-05-31T08:23:08-04:00S-shaped current-voltage characteristics of organic solar devicesMeasuring the current-voltage characteristic of organic bulk heterojunction solar devices sometimes reveals an s-shaped deformation. We qualitatively produce this behaviour by a numerical device simulation assuming a reduced surface recombination. Furthermore we show how to experimentally create these double diodes by applying an oxygen plasma etch on the indium tin oxide (ITO) anode. Restricted charge transport over material interfaces accumulates space charges and therefore creates s-shaped deformations. Finally we discuss the consequences of our findings for the open circuit voltage $V_{oc}$A. WagenpfahlD. RauhM. BinderC. DeibelV. Dyakonov10.1103/PhysRevB.82.1153069 pages, 7 figuresPhys. Rev. B 82, 115306 (2010)http://arxiv.org/abs/1003.0359v42010-08-30T03:06:25-04:002010-03-01T09:36:09-05:00Polymer-Fullerene Bulk Heterojunction Solar CellsOrganic solar cells have the potential to be low-cost and efficient solar energy converters, with a promising energy balance. They are made from carbon-based semiconductors, which exhibit favourable light absorption and charge generation properties, and can be manufactured by low temperature processes such as printing from solvent-based inks, which are compatible with flexible plastic substrates or even paper. In this review, we will present an overview of the physical function of organic solar cells, their state-of-the-art performance and limitations, as well as novel concepts to achieve a better material stability and higher power conversion efficiencies. We will also briefly review processing and cost in view of the market potential.Carsten DeibelVladimir Dyakonov10.1088/0034-4885/73/9/09640168 pages, 16 figures. Minor changesRep. Prog. Phys. 73, 096401 (2010)http://arxiv.org/abs/1008.4230v12010-08-25T04:43:23-04:002010-08-25T04:43:23-04:00Oxygen doping of P3HT:PCBM blends: Influence on trap states, charge carrier mobility and solar cell performanceWe investigated the influence of oxygen on the performance of P3HT:PCBM (poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester) solar cells by current--voltage, thermally stimulated current (TSC) and charge extraction by linearly increasing voltage (CELIV) measurement techniques. The exposure to oxygen leads to an enhanced charge carrier concentration and a decreased charge carrier mobility. Further, an enhanced formation of deeper traps was observed, although the overall density of traps was found to be unaffected upon oxygen exposure. With the aid of macroscopic simulations, based on solving the differential equation system of Poisson, continuity and drift-diffusion equations in one dimension, we demonstrate the influence of a reduced charge carrier mobility and an increased charge carrier density on the main solar cell parameters, consistent with experimental findings.Julia SchafferhansAndreas BaumannAlexander WagenpfahlCarsten DeibelVladimir Dyakonov10.1016/j.orgel.2010.07.016Organic Electronics 11, 1693 (2010)http://arxiv.org/abs/1007.1653v12010-07-09T15:39:01-04:002010-07-09T15:39:01-04:00Spectroscopic Signatures of Photogenerated Radical Anions in Polymer-[C70]Fullerene Bulk HeterojunctionsLight induced polarons in solid films of polymer-fullerene blends were studied by applying photoluminescence (PL), photo induced absorption (PIA) techniques as well as electron spin resonance (ESR). The materials used were poly(3-hexylthiophene) (P3HT) and poly-[2-methoxy, 5-(2'-ethyl-hexyloxy) phenylene vinylene] (MEH-PPV) as donors. As acceptors we used [6,6]-phenyl-C61-butyric acid methyl ester ([C60]PCBM) and various soluble C70-derivates: [C70]PCBM, diphenylmethano[70]fullerene oligoether (C70-DPM-OE), C70-DPM-OE2, and two fullerene dimers, C70-C70 and C60-C70 (all shown in figure 1). In all blends containing C70 we found typical signatures which were absent if [C60]PCBM was used as acceptor. Light-induced ESR revealed signals at g>=2.005, which we previously assigned to an electron localized on the C70 cage, the PIA measurements showed a new sub-bandgap absorption band at 0.92 eV, which we correspondingly ascribe to C70 radical anions formed in the course of photoinduced electron transfer from donor to acceptor.M. LiedtkeA. SperlichH. KrausC. DeibelV. DyakonovS. FilipponeJ. L. DelgadoN. MartinO. G. Poluektov10.1149/1.35033474 pages, 5 figuresMeet. Abstr. - Electrochem. Soc. 1001, 1494 (2010)http://arxiv.org/abs/1006.4971v12010-06-25T08:03:03-04:002010-06-25T08:03:03-04:00Effect of doping of zinc oxide on the hole mobility of poly(3-hexylthiophene) in hybrid transistorsHybrid field effect transistors based on the organic polymer poly(3-hexylthiophene) (P3HT) and inorganic zinc oxide were investigated. In this report we present one of the first studies on hybrid transistors employing one polymeric transport layer. The sol-gel processed ZnO was modified via Al doping between 0.8 and 10 at.%, which allows a systematic variation of the zinc oxide properties, i.e. electron mobility and morphology. With increasing doping level we observe on the one hand a decrease of the electron mobilities by two orders of magnitude,on the other hand doping enforces a morphological change of the zinc oxide layer which enables the infiltration of P3HT into the inorganic matrix. X-ray reflectivity (XRR) measurements confirm this significant change in the interface morphology for the various doping levels. We demonstrate that doping of ZnO is a tool to adjust the charge transport in ZnO/P3HT hybrids, using one single injecting metal (Au bottom contact) on a SiO2 dielectric. We observe an influence of the zinc oxide layer on the hole mobility in P3HT which we can modify via Al doping of ZnO. Hence, maximum hole mobility of 5e-4 cm^2/Vs in the hybrid system with 2 at.% Al doping. 5 at.% Al doping leads to a balanced mobility in the organic/inorganic hybrid system but also to a small on/off ratio due to high off-currents.Maria S. HammerCarsten DeibelJens PflaumVladimir Dyakonov10.1016/j.orgel.2010.06.01915 pages, 8 figures, accepted for publication in Organic ElectronicsOrg. Electron. 11, 1569 (2010)http://arxiv.org/abs/1002.3568v12010-02-18T11:41:48-05:002010-02-18T11:41:48-05:00Polymeric Squaraine Dyes as Electron Donors in Bulk Heterojunction Solar CellsA polysquaraine low band gap polymer was synthesized by Yamamoto coupling of a monomeric dibromo indolenine squaraine dye. The resulting polymer has a weight average molar mass in the order of Mw ~30.000-50.000 and a polydispersity of ca. 1.7 as determined by gel-permeation chromatography (GPC). The electronic properties of monomer and polymer were investigated by cyclic voltammetry, absorption and emission spectroscopy. Owing to exciton coupling the absorption bands of the polymer are red-shifted and strongly broadened compared to the monomer squaraine dye. Bulk heterojunction solar cells were prepared from blends of the polysquaraine with the fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester (PCBM) in different weight ratios (1:3 to 1:1). The power conversion efficiencies under simulated AM 1.5 conditions yielded 0.45 % for these non-optimized systems. The external quantum efficiency (EQE) shows that the photoresponse spans the range from 300 to 850 nm, which illustrates the promising properties of this novel organic semiconductor as a low band gap donor material in organic photovoltaics.Sebastian F. VoelkerShinobu UemuraMoritz LimpinselMarkus MingebachCarsten DeibelVladimir DyakonovChristoph Lambert10.1002/macp.20090067010 pages, 7 figuresMacromol. Chem. Phys. 211, 1098 (2010)http://arxiv.org/abs/1001.4963v22010-02-16T04:18:40-05:002010-01-27T10:14:36-05:00Organic solar cell efficiencies under the aspect of reduced surface recombination velocitiesThe charge carrier mobility is a key parameter for the organic bulk heterojunction solar cell efficiency. It was recently shown that the interplay charge carrier transport and recombination, both depending on electron and hole mobilities, leads to a point of maximum power conversion efficiency at a finite mobility. Changes of bulk and surface recombination rate, however, can strongly influence this behavior. These processes were previously not considered adequately, as surface recombination velocities of infinity were implicitly assumed or bulk recombination parameters not discussed in detail. In this manuscript, using a macroscopic effective medium simulation, we consider how a reduced bulk recombination process in combination with finite surface recombination velocities affect the power conversion efficiency. Instead of a maximum efficiency at a specific charge carrier mobility, we show that with realistic assumptions and passivated surfaces the efficiency is increased further, saturating only at higher mobilities. Thus, a mobility optimisation is more important for the solar cell performance then previously shown.A. WagenpfahlC. DeibelV. Dyakonov10.1109/JSTQE.2010.20421426 pages, 5 figuresIEEE J. Sel. Top. Quantum Electron. 16, 1759 (2010)http://arxiv.org/abs/1001.2546v12010-01-14T15:01:36-05:002010-01-14T15:01:36-05:00Investigation of the Photocurrent in Bulk Heterojunction Solar CellsWe investigated the photocurrent in poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-phenyl-C$_{61}$ butyric acid methyl ester (PCBM) solar cells by applying a pulsed measurement technique. For annealed samples, a point of optimal symmetry (POS) with a corresponding voltage $V_\text{POS}$ of 0.52--0.64 V could be determined. Based on macroscopic simulations and results from capacitance--voltage measurements, we identify this voltage with flat band conditions in the bulk of the cell, but not the built-in voltage as proposed by [Ooi et al., J. Mater. Chem. 18 (2008) 1644]. We calculated the field dependent polaron pair dissociation after Onsager--Braun and the voltage dependent extraction of charge carriers after Sokel and Hughes with respect to this point of symmetry. Our analysis allows to explain the experimental photocurrent in both forward and reverse directions. Also, we observed a voltage--independent offset of the photocurrent. As this offset is crucial for the device performance, we investigated its dependence on cathode material and thermal treatment. From our considerations we gain new insight into the photocurrent`s voltage dependence and the limitations of device efficiency.M. LimpinselA. WagenpfahlM. MingebachC. DeibelV. Dyakonov10.1103/PhysRevB.81.0852036 pages, 7 figuresPhys. Rev. B 81, 085203 (2010)http://arxiv.org/abs/1001.2330v12010-01-13T17:47:49-05:002010-01-13T17:47:49-05:00Energetics of Excited States in the Conjugated Polymer Poly(3-hexylthiophene)There is an enormous potential in applying conjugated polymers in novel organic opto-electronic devices such as light emitting diodes and solar cells. Although prototypes and first products exist, a comprehensive understanding of the fundamental processes and energetics involved during photoexcitation is still lacking and limits further device optimisations. Here we report on a unique analysis of the excited states involved in charge generation by photoexcitation. On the model system poly(3-hexylthiophene) (P3HT), we demonstrate the general applicability of our novel approach. From photoemission spectroscopy of occupied and unoccupied states we determine the transport gap to 2.6 eV, which we show to be in agreement with the onset of photoconductivity by spectrally resolved photocurrent measurements. For photogenerated singlet exciton at the absorption edge, 0.7 eV of excess energy are required to overcome the binding energy; the intermediate charge transfer state is situated only 0.3 eV above the singlet exciton. Our results give direct evidence of energy levels involved in the photogeneration and charge transport within conjugated polymers.Carsten DeibelDaniel MackJulien GorenflotAchim SchoellStefan KrauseFriedrich ReinertDaniel RauhVladimir Dyakonov10.1103/PhysRevB.81.0852025 pages, 4 figuresPhys. Rev. B 81, 085202 (2010)http://arxiv.org/abs/0912.1336v12009-12-07T17:28:24-05:002009-12-07T17:28:24-05:00Improvement of the poly-3-hexylthiophene transistor performance using small molecule contact functionalizationWe demonstrate an approach to improve poly-3-hexylthiophene field effect transistors by modifying the gold contacts with monolayer thick pentacenequinone (PQ) or naphthalene (NL). The effective contact resistance is reduced by a factor of two and sixteen for interlayers of PQ and NL, respectively. The observation is attributed to different injection barriers at the metal-organic interface caused by the functionalization and to an additional tunneling barrier enhancing the on/off ratios. This barrier yields to activation energies of 37meV (NL) and 104meV (PQ) below 190K, which are smaller than without functionalization, 117meV.Rebecca WinterMaria S HammerCarsten DeibelJens Pflaum10.1063/1.32766944 pages, 3 figures, 1 table; accepted for publication by Appl. Phys. LettAppl. Phys. Lett. 95, 263313 (2009)http://arxiv.org/abs/0906.2486v22009-07-16T11:30:25-04:002009-06-13T12:06:24-04:00Origin of the Efficient Polaron Pair Dissociation in Polymer--Fullerene BlendsThe separation of photogenerated polaron pairs in organic bulk heterojunction solar cells is the intermediate but crucial step between exciton dissociation and charge transport to the electrodes. In state-of-the-art devices, above 80% of all polaron pairs are separated at fields of below $10^7$V/m. In contrast, considering just the Coulomb binding of the polaron pair, electric fields above $10^8$V/m would be needed to reach similar yields. In order to resolve this discrepancy, we performed kinetic Monte Carlo simulations of polaron pair dissociation in donor--acceptor blends, considering delocalised charge carriers along conjugated polymer chain segments. We show that the resulting fast local charge carrier transport can indeed explain the high experimental quantum yields in polymer solar cells.C. DeibelT. StrobelV. Dyakonov10.1103/PhysRevLett.103.0364024 pages, 2 figures. Changes: text shortened during proof stagePhys. Rev. Lett. 103, 036402 (2009)http://arxiv.org/abs/0907.2559v12009-07-15T07:14:40-04:002009-07-15T07:14:40-04:00Polaron Recombination in Pristine and Annealed Bulk Heterojunction Solar CellsThe major loss mechanism of photogenerated polarons was investigated in P3HT:PCBM solar cells by the photo-CELIV technique. For pristine and annealed devices, we find that the experimental data can be explained by a bimolecular recombination rate reduced by a factor of about ten (pristine) and 25 (annealed) as compared to Langevin theory. Aided by a macroscopic device model, we discuss the implications of the lowered loss rate on the characteristics of polymer:fullerene solar cells.C. DeibelA. BaumannA. WagenpfahlV. Dyakonov10.1016/j.synthmet.2009.07.0143 pages, 4 figuresSynth. Met. 159, 2345 (2009)http://arxiv.org/abs/0907.2294v12009-07-14T15:08:03-04:002009-07-14T15:08:03-04:00Charge Carrier Dissociation and Recombination in Polymer Solar CellsIn polymer:fullerene solar cells, the origin of the losses in the field-dependent photocurrent is still controversially debated. We contribute to the ongoing discussion by performing photo-induced charge extraction measurements on poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C$_{61}$ butyric acid methyl ester solar cells in order to investigate the processes ruling charge carrier decay. Calculating the drift length of photogenerated charges, we find that polaron recombination is not limiting the photocurrent for annealed devices. Additionally, we applied Monte Carlo simulations on blends of conjugated polymer chain donors with acceptor molecules in order to gain insight into the polaron pair dissociation. The dissociation yield turns out to be rather high, with only a weak field dependence. With this complementary view on dissociation and recombination, we stress the importance of accounting for polaron pair dissociation, polaron recombination as well as charge extraction when considering the loss mechanisms in organic solar cells.C. Deibel10.1002/pssa.20092526 pages, 7 figuesPhys. Status Solidi A, 206, 2731 (2009)http://arxiv.org/abs/0907.2428v12009-07-14T15:01:46-04:002009-07-14T15:01:46-04:00Origin of Reduced Polaron Recombination in Organic Semiconductor DevicesWe propose a model to explain the reduced bimolecular recombination rate found in state-of-the-art bulk heterojunction solar cells. When compared to the Langevin recombination, the experimentally observed rate is one to four orders of magnitude lower, but gets closer to the Langevin case for low temperatures. Our model considers the organic solar cell as device with carrier concentration gradients, which form due to the electrode/blend/electrode device configuration. The resulting electron concentration under working conditions of a solar cell is higher at the cathode than at the anode, and vice versa for holes. Therefore, the spatially dependent bimolecular recombination rate, proportional to the local product of electron and hole concentration, is much lower as compared to the calculation of the recombination rate based on the extracted and thus averaged charge carrier concentrations. We consider also the temperature dependence of the recombination rate, which can for the first time be described with our model.C. DeibelA. WagenpfahlV. Dyakonov10.1103/PhysRevB.80.0752037 pages, 6 figuresPhys. Rev. B 80, 075203 (2009)http://arxiv.org/abs/0907.1401v12009-07-08T08:56:01-04:002009-07-08T08:56:01-04:00Charge Carrier Concentration and Temperature Dependent Recombination in Polymer Fullerene Solar CellsWe performed temperature dependent transient photovoltage and photocurrent measurements on poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methylester bulk heterojuction solar cells. We found a strongly charge carrier concentration and temperature dependent Langevin recombination prefactor. The observed recombination mechanism is discussed in terms of bimolecular recombination. The experimental results were compared with charge carrier extraction by linearly increasing voltage (photo-CELIV) measurements done on the same blend system. We explain the charge carrier dynamics, following an apparent order larger than two, by dynamic trapping of charges in the tail states of the gaussian density of states.A. FoertigA. BaumannD. RauhV. DyakonovC. Deibel10.1063/1.32023894 pages, 3 figures, 1 tableAppl. Phys. Lett. 95, 052104 (2009)http://arxiv.org/abs/0810.5460v32008-12-23T08:43:07-05:002008-10-30T08:07:27-04:00Bipolar Charge Transport in Poly(3-hexyl thiophene)/Methanofullerene Blends: A Ratio Dependent StudyWe investigated the charge carrier mobility in pristine poly(3-hexyl thiophene-2,5-diyl) (P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend devices by applying the time resolved photoconductivity experiment in dependence on the donor:acceptor ratio. We observe a bipolar transport in all studied samples ranging from pure polymer to polymer:fullerene with 90% PCBM content. For the ratios P3HT:PCBM 1:4 and 1:1 we observe two transit times in the electron current transients, as well as hole double transients for P3HT:PCBM 1:2. We find high hole and electron mobilities in the order of 10^(-3) - 10^(-2) cm^2/Vs for a concentration of 90% PCBM in the blend.A. BaumannJ. LorrmannC. DeibelV. Dyakonov10.1063/1.30556083 pages, 1 table, 2 figures, minor corrections includedApplied Physics Letters 93, 252104 (2008)http://arxiv.org/abs/0811.1924v12008-11-12T10:57:27-05:002008-11-12T10:57:27-05:00Effect of doping-- and field--induced charge carrier density on the electron transport in nanocrystalline ZnOCharge transport properties of thin films of sol--gel processed undoped and Al-doped zinc oxide nanoparticles with variable doping level between 0.8 at% and 10 at% were investigated. The X-ray diffraction studies revealed a decrease of the average crystallite sizes in highly doped samples. We provide estimates of the conductivity and the resulting charge carrier densities with respect to the doping level. The increase of charge carrier density due to extrinsic doping were compared to the accumulation of charge carriers in field effect transistor structures. This allowed to assess the scattering effects due to extrinsic doping on the electron mobility. The latter decreases from 4.6*10^-3 cm^2/Vs to 4.5*10^-4 cm^2/Vs with increasing doping density. In contrast, the accumulation leads to an increasing mobility up to 1.5*10^-2 cm^2/Vs. The potential barrier heights related to grain boundaries between the crystallites were derived from temperature dependent mobility measurements. The extrinsic doping initially leads to a grain boundary barrier height lowering, followed by an increase due to doping-induced structural defects. We conclude that the conductivity of sol--gel processed nanocrystalline ZnO:Al is governed by an interplay of the enhanced charge carrier density and the doping-induced charge carrier scattering effects, achieving a maximum at 0.8 at% in our case.Maria S HammerDaniel RauhCarsten DeibelVladimir Dyakonov10.1088/0957-4484/19/48/4857018 pages, 7 figuresNanotechnology 19 (2008) 485701http://arxiv.org/abs/0810.0542v22008-10-30T06:52:34-04:002008-10-02T18:00:28-04:00Polaron Recombination in Pristine and Annealed Bulk Heterojunction Solar CellsWe determined the dominant polaron recombination loss mechanism in pristine and annealed polythiophene:fullerene blend solar cells by applying the photo-induced charge extraction by linearly increasing voltage (photo-CELIV) method in dependence on temperature. In pristine samples, we find a strongly temperature dependent bimolecular polaron recombination rate, which is reduced as compared to the Langevin theory. For the annealed sample, we observe a polaron decay rate which follows a third order of carrier concentration almost temperature independently.C. DeibelA. BaumannV. Dyakonov10.1063/1.30055934 pages, 3 figuresAppl. Phys. Lett. 93, 163303 (2008)http://arxiv.org/abs/0810.3534v12008-10-20T07:43:54-04:002008-10-20T07:43:54-04:00Trap Distribution and the Impact of Oxygen-induced Traps on the Charge Transport in Poly(3-Hexylthiophene)The trap distribution in the conjugated polymer poly(3-hexylthiophene) was investigated by fractional thermally stimulated current measurements. Two defect states with activation energies of about 50 meV and 105 meV and Gaussian energy distributions were revealed. The first is assigned to the tail of the intrinsic density of states, whereas the concentration of second trap is directly related to oxygen exposure. The impact of the oxygen induced traps on the charge transport was examined by performing photo-induced charge carrier extraction by linearly increasing voltage measurements, that exhibited a strong decrease in the mobility with air exposure time.Julia SchafferhansExperimental Physics VI, Faculty of Physics and Astronomy, Julius-Maximilians-University of Würzburg, Würzburg, GermanyAndreas BaumannExperimental Physics VI, Faculty of Physics and Astronomy, Julius-Maximilians-University of Würzburg, Würzburg, GermanyCarsten DeibelExperimental Physics VI, Faculty of Physics and Astronomy, Julius-Maximilians-University of Würzburg, Würzburg, GermanyVladimir DyakonovExperimental Physics VI, Faculty of Physics and Astronomy, Julius-Maximilians-University of Würzburg, Würzburg, GermanyZAE Bayern, Bavarian Center of Applied Energy Research, Würzburg, Germany10.1063/1.29782374 pages, 3 figures, 2 tablesApplied Physics Letters 93, 093303 (2008)http://arxiv.org/abs/0806.2249v22008-06-18T05:47:19-04:002008-06-13T09:31:32-04:00Influence of Charge Carrier Mobility on the Performance of Organic Solar CellsThe power conversion efficiency of organic solar cells based on donor--acceptor blends is governed by an interplay of polaron pair dissociation and bimolecular polaron recombination. Both processes are strongly dependent on the charge carrier mobility, the dissociation increasing with faster charge transport, with raised recombination losses at the same time. Using a macroscopic effective medium simulation, we calculate the optimum charge carrier mobility for the highest power conversion efficiency, for the first time accounting for injection barriers and a reduced Langevin-type recombination. An enhancement of the charge carrier mobility from $10^{-8}$m$^2$/Vs for state of the art polymer:fullerene solar cells to about $10^{-6}$m$^2$/Vs, which yields the maximum efficiency, corresponds to an improvement of only about 20% for the given parameter set.C. DeibelA. WagenpfahlV. Dyakonov10.1002/pssr.2008021103 pages, 4 figuresphys.stat.sol. (RRL) 2, 175 (2008)http://arxiv.org/abs/0805.0221v12008-05-02T08:40:58-04:002008-05-02T08:40:58-04:00Sonnenstrom aus PlastikInspired by the success of organic light emitting diodes, organic solar cells have the potential to establish interesting applications, complementing and enhancing the application range of inorganic systems.
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Durch den Erfolg organischer Leuchtdioden befl\"ugelt, entwickeln sich auch organische Solarzellen zu interessanten Anwendungen, welche das Potential haben, die Anwendungsbereiche anorganischer Systeme zu erg\"anzen und erweitern.Carsten DeibelVladimir Dyakonov4 pages, in germanPhysik Journal 7:5 (2008) 51http://arxiv.org/abs/0712.2982v12007-12-18T10:16:23-05:002007-12-18T10:16:23-05:00Spectroscopy of electronic defect states in Cu(In, Ga)(S, Se)$_2$-based heterojunctions and Schottky diodes under damp-heat exposureThe changes of defect characteristics induced by accelerated lifetime tests on the heterostructure n-ZnO/i-ZnO/CdS/Cu(In, Ga)(S, Se)$_2$/Mo relevant for photovoltaic energy conversion are investigated. We subject heterojunction and Schottky devices to extended damp heat exposure at 85$^{\circ}$C ambient temperature and 85% relative humidity for various time periods. In order to understand the origin of the pronounced changes of the devices, we apply current--voltage and capacitance--voltage measurements, admittance spectroscopy, and deep-level transient spectroscopy. The fill factor and open-circuit voltage of test devices are reduced after prolonged damp heat treatment, leading to a reduced energy conversion efficiency. We observe the presence of defect states in the vicinity of the CdS/chalcopyrite interface. Their activation energy increases due to damp heat exposure, indicating a reduced band bending at the Cu(In, Ga)(S, Se)$_2$ surface. The Fermi-level pinning at the buffer/chalcopyrite interface, maintaining a high band bending in as-grown cells, is lifted due to the damp-heat exposure. We also observe changes in the bulk defect spectra due to the damp-heat treatment.C. DeibelV. DyakonovJ. Parisi10.1209/epl/i2003-10211-34 pages, 5 figuresEurophysics Letters, 66 (2004) 399-404http://arxiv.org/abs/0712.2502v12007-12-15T04:05:08-05:002007-12-15T04:05:08-05:00Band Bending Independent of Surface Passivation in ZnO/CdS/Cu(In,Ga)(S,Se)$_2$ Heterojunctions and Cr/Cu(In,Ga)(S,Se)$_2$ Schottky ContactsWe have employed admittance spectroscopy and deep-level transient spectroscopy in order to investigate the electronic properties of ZnO/CdS/Cu(In,Ga)(S,Se)$_2$ heterojunctions and Cr/Cu(In,Ga)(S,Se)$_2$ Schottky contacts. Our work concentrates on the origin of an energy-distributed defect state commonly found in these systems. The activation energy of the defect state addressed continuously shifts upon air annealing or damp-heat treatment and is a valuable measure of the degree of band bending in Cu(In,Ga)(S,Se)$_2$-based junctions. We demonstrate that the band bending within the Cu(In,Ga)(S,Se)$_2$ layer, reported in the literature to become minimal after air exposure, returns after the formation of either a Schottky contact or a heterojunction. The earlier phenomenon turns out to be independent of a surface passivation due to the CdS bath deposition.C. DeibelV. DyakonovJ. Parisi10.1063/1.15765003 pages, 3 figuresApplied Physics Letters 82 (2003) 3559-3561http://arxiv.org/abs/cond-mat/0607392v12006-07-14T18:47:07-04:002006-07-14T18:47:07-04:00Charge Transport Properties of a Metal-free Phthalocyanine Discotic Liquid CrystalDiscotic liquid crystals can self-align to form one-dimensional semiconducting wires, many tens of microns long. In this letter, we describe the preparation of semiconducting films where the stacking direction of the disc-like molecules is perpendicular to the substrate surface. We present measurements of the charge carrier mobility, applying temperature-dependent time-of-flight transient photoconductivity, space-charge limited current measurements, and field-effect mobility measurements. We provide experimental verification of the highly anisotropic nature of semiconducting films of discotic liquid crystals, with charge carrier mobilities of up to 2.8x10$^{-3}$cm$^2$/Vs. These properties make discotics an interesting choice for applications such as organic photovoltaics.C. DeibelD. JanssenP. HeremansV. de CupereY. GeertsM. L. BenkhedirG. J. Adriaenssens10.1016/j.orgel.2006.07.0025 pages, 5 figuresOrganic Electronics 7 (2006) 495-499